A respiratory system with a flow sensor has become known from DE-A1-34,34,908. The respiratory system contains nonreturn valves as final control elements and direction valves which control the flow of the respiration gas stream. Fresh gas is fed into the respiratory system via a fresh gas connection and a first nonreturn valve. On the inspiration side, the respiration gas flows through a carbon dioxide absorber, a first directional valve and a pressure sensor and to the lung of a patient; on the expiration side, there are a flow sensor and a second directional valve, via which the respiration gas is returned into the inspiration arm. A second nonreturn valve for releasing excess respiration gas and a drive unit with variable volume are provided in the respiratory system, and the drive unit feeds respiration gas to the patient's lung in a rhythmic sequence. The course of respiration over time is determined by a control unit.
Different forms of respiration can be realized by correspondingly actuating the nonreturn valves. A so-called closed respiratory system is obtained when only as much fresh gas is metered into the respiratory system as is consumed by the patient, while in a semi-closed respiratory system, one operates with excess fresh gas and the excess respiration gas is released from the respiratory system after each breath.
Respiration is monitored with a pressure sensor for the respiration pressure and a flow sensor which measures the volume expired by the patient. One widely used embodiment of a flow sensor is based on constant-temperature hot wire anemometry. This measurement method depends on the type of the gas, on the one hand, and, on the other hand, a reference value for "zero" flow must be available in the measuring device. Concerning correction for the effect of the type of the gas, compensation methods are known which are able to take binary gas mixtures containing known components, e.g., laughing gas and oxygen, adequately into account. However, these compensation methods are unable to evaluate gas mixtures containing more than two components. Three components are present, e.g., when anesthetic is also metered into the respiratory system in addition to oxygen and laughing gas. The influence of the anesthetic on flow measurement is negligible if anesthetics to be metered at low doses are used, at concentrations of up to ca. 4%.
In contrast, the accuracy of measurement of the flow sensor can be expected to be reduced in the case of anesthetics to be metered at high doses (up to 20%).
The reference value for "zero" flow is usually obtained by turning off the drive unit and measuring the reference value with the respiration gas stream stopped. However, it is also possible to determine this reference value during the reversal pause between inspiration and expiration.